In conventional tire air pressure detecting systems a direct-type tire air pressure detecting systems has a transmitter, which is installed in a wheel having a tire and has a sensing unit such as a pressure sensor. The detector also has a receiver which has an antenna and is installed in a chassis of a vehicle. When the transmitter transmits a radio wave including a detection signal received from the sensing unit, the receiver receives the wave by means of the antenna. Thus, air pressure of the tire is detected.
As such a tire air pressure detecting system, a batteryless direct-type tire air pressure detecting system is desired. In this system, a transmitter does not have a battery for transmission and is electrically charged by a transponder method, which is widely known (JP 2000-255229A). In such a system using transponder method, an onboard receiver transmits an electrical wave for electrical charging (hereafter charging wave) through an onboard antenna and a capacitor installed in the transmitter is charged by the charging wave. When the capacitor stores sufficient electrical power, the transmitter transmits a detection signal.
In the batteryless tire air pressure detecting system, the power of the electrical charging wave is strong, while the power of the wave including the detection signal (hereafter response wave) from the transmitter to the receiver is weak. It is assumed here that two vehicles have the same batteryless tire air pressure detecting systems and come close to each other as shown in FIG. 8. In this situation, if a receiver J2 in one of the vehicles (hereafter the first vehicle) transmits a charging wave while a transmitter J1 in the other vehicle (hereafter the second vehicle) is transmitting a response wave, the response wave is jammed by the charging wave.
The transmission between the transmitter and the receiver may get frequent, because the batteryless tire air pressure detecting system does not have to save the consumption of battery and there may be a need of the frequent transmission in order to control the vehicle according to the detected tire air pressure. In this case it becomes necessary to find a solution to the problem.